1. Field of the Invention
An aspect of the present invention relates to a polymer electrolyte membrane for use with a fuel cell and a method of producing the same, and, more particularly, to a fuel cell polymer electrolyte membrane having relatively excellent mechanical strength and a substantially improved liquid holding capability and a method of producing the same.
2. Description of the Related Art
A group of fuel cells form an energy generating system in which a chemical reaction energy between oxygen, and hydrogen of a hydrocarbon-based material, such as methanol, ethanol, or natural gas, is directly converted into an electrical energy. According to an electrolyte that is used, fuel cells may be categorized into phosphoric acid type fuel cells, molten carbonate type fuel cells, solid oxide type fuel cells, polymer electrolyte membrane fuel cells (PEMFCs), alkali type fuel cells, and the like. These fuel cells operate based on similar principles, but have different fuels, different operating temperatures, different catalysts, different electrolytes, etc.
Among these fuel cells, PEMFCs, which have recently been developed, have better energy output properties, lower operating temperatures, faster start-ups, and quicker response times than the other fuel cells. Due to these advantages, the PEMFC has a wide range of applications, such as being portable power sources for cars, individual power sources for homes or buildings, and small power sources for electronic devices.
Conventionally, a PEMFC includes a polymer electrolyte membrane having a polymer electrolyte, such as a perfluorosulfonate polymer (for example, NAFION produced from Dupont Inc.) that has a main chain of an alkylene fluoride and a side chain of vinyl ether fluoride terminated with a sulfonic acid group. It is noteworthy that the polymer electrolyte membrane has high ionic conductivity by holding a proper amount of water must be considered.
In order to prevent dryness of the polymer electrolyte membrane of the PEMFC, the conventional PEMFC operates at 100° C. or less, for example, at approximately 80° C. However, operating at such low temperatures results in problems which will now be described. A hydrogen-rich gas, which is mainly used as a fuel for a PEMFC, may be obtained by reforming an organic fuel, such as natural gas or methanol. In this case, however, the hydrogen-rich gas contains CO as well as CO2 as a by-product. The CO poisons catalysts contained in a cathode and an anode. When a catalyst is poisoned with CO, its electrochemical activity decreases significantly. Thus, the operation efficiency and lifetime of the PEMFC decrease significantly. Moreover, the catalyst is more readily poisoned when the operating temperature of the PEMFC is lower.
Meanwhile, when the operating temperature of the PEMFC is increased to about 150° C. or higher, the poisoning of the catalyst due to CO may be avoided and the temperature of the PEMFC may be easily controlled. As a result, a fuel reformer may be miniaturized and a cooling device may be simplified. Thus, the entire energy generating system of a PEMFC may be miniaturized. However, a conventional electrolyte membrane, that is, a polymer electrolyte membrane composed of, for example, a perfluoro sulfonate polymer (for example, NAFION produced from Dupont Inc.) that has a main chain of a alkylene fluoride and a side chain of vinyl ether fluoride terminated with a sulfonic acid group, experiences a significant drop in performance due to evaporation of moisture at such a high temperature. In addition, a polymer containing a sulfonic acid group fails to maintain its original form at about 120° C. or higher. As a result, the polymer electrolyte membrane formed using the perfluoro sulfonate polymer cannot act as an electrolyte membrane at such high temperatures.
In order to solve this problem, non-humidified polymer electrolytes that operate at high temperatures have been actively researched mainly based on a polybenzimidazole (PBI)-phosphoric acid system that uses a phosphoric acid (H3PO4) as a proton conductor.
The PBI-phosphoric acid system swells by holding a phosphoric acid in a polymer matrix, and repeatedly shrinks and swells during use. However, since a PBI matrix is susceptible to shrinking and swelling due to its relatively small mechanical strength, the PBI matrix is easily broken or damaged. In a conventional PBI-phosphoric acid system, an ortho-phosphoric acid dissolves in water generated by reactions between hydrogen ions and oxygen and leaks. Thus, the ionic conductivity of an electrolyte membrane decreases, and, when a fuel cell operates for a long time at a high temperature, the polymer matrix dissolves in the phosphoric acid. In other words, at high temperature, the polymer matrix loses water through a condensation reaction of phosphoric acid molecules and forms a polyphosphoric acid. The formed polyphosphoric acid decreases ionic conductivity and dissolves a polymer electrolyte membrane.